This invention generally relates to electrical drivers for a matrix of current driven devices, and more particularly to methods and apparatus for determining and providing a precharge for such devices.
There is a great deal of interest in xe2x80x9cflat panelxe2x80x9d displays, particularly for small to midsized displays, such as may be used in laptop computers, cell phones, and personal digital assistants. Liquid crystal displays (LCDs) are a well-known example of such flat panel video displays, and employ a matrix of xe2x80x9cpixelsxe2x80x9d which selectably block or transmit light. LCDs do not provide their own light; rather, the light is provided from an independent source. Moreover, LCDs are operated by an applied voltage, rather than by current. Luminescent displays are an alternative to LCD displays. Luminescent displays produce their own light, and hence do not require an independent light source. They typically include a matrix of elements which luminesce when excited by current flow. A common luminescent device for such displays is a light emitting diode (LED).
LED arrays produce their own light in response to current flowing through the individual elements of the array. The current flow may be induced by either a voltage source or a current source. A variety of different LED-like luminescent sources have been used for such displays. The embodiments described herein utilize organic electroluminescent materials in OLEDs (organic light emitting diodes), which include polymer OLEDs (PLEDs) and small-molecule OLEDs, each of which is distinguished by the molecular structure of their color and light producing material as well as by their manufacturing processes. Electrically, these devices look like diodes with forward xe2x80x9conxe2x80x9d voltage drops ranging from 2 volts (V) to 20 V depending on the type of OLED material used, the OLED aging, the magnitude of current flowing through the device, temperature, and other parameters. Unlike LCDs, OLEDs are current driven devices; however, they may be similarly arranged in a 2 dimensional array (matrix) of elements to form a display.
OLED displays can be either passive-matrix or active-matrix. Active-matrix OLED displays use current control circuits integrated with the display itself, with one control circuit corresponding to each individual element on the substrate, to create high-resolution color graphics with a high refresh rate. Passive-matrix OLED displays are easier to build than active-matrix displays, because their current control circuitry is implemented external to the display. This allows the display manufacturing process to be significantly simplified.
FIG. 1A is an exploded view of a typical physical structure of such a passive-matrix display 100 of OLEDs. A layer 110 having a representative series of rows, such as parallel conductors 111-118, is disposed on one side of a sheet of light emitting polymer, or other emissive material, 120. A representative series of columns are shown as parallel transparent conductors 131-138, which are disposed on the other side of sheet 120, adjacent to a glass plate 140. FIG. 1B is a cross-section of the display 100, and shows a drive voltage V applied between a row 111 and a column 134. A portion of the sheet 120 disposed between the row 111 the column 134 forms an element 150 which behaves like an LED. The potential developed across this LED causes current flow, so the LED emits light 170. Since the emitted light 170 must pass through the column conductor 134, such column conductors are transparent. Most such transparent conductors have relatively high resistance compared with the row conductors 111-118, which may be formed from opaque materials, such as copper, having a low resistivity.
This structure results in a matrix of devices, one device formed at each point where a row overlies a column. There will generally be Mxc3x97N devices in a matrix having M rows and N columns. Typical devices function like light emitting diodes (LEDs), which conduct current and luminesce when voltage of one polarity is imposed across them, and block current when voltage of the opposite polarity is applied. Exactly one device is common to both a particular row and a particular column, so to control these individual LED devices located at the matrix junctions it is useful to have two distinct driver circuits, one to drive the columns and one to drive the rows. It is conventional to sequentially scan the rows (conventionally connected to device cathodes) with a driver switch to a known voltage such as ground, and to provide another driver, which may be a current source, to drive the columns (which are conventionally connected to device anodes).
FIG. 2 represents such a conventional arrangement for driving a display having M rows and N columns. A column driver device 260 includes one column drive circuit (e.g. 262, 264, 266) for each column. The column driver circuit 264 shows some of the details which are typically provided in each column driver, including a current source 270 and a switch 272 which enables a column connection 274 to be connected to either the current source 270 to illuminate the selected diode, or to ground to turn off the selected diode. A scan circuit 250 includes representations of row driver switches (208, 218, 228, 238 and 248). A luminescent display 280 represents a display having M rows and N columns, though only five representative rows and three representative columns are drawn.
The rows of FIG. 2 are typically a series of parallel connection lines traversing the back of a polymer, organic or other luminescent sheet, and the columns are a second series of connection lines perpendicular to the rows and traversing the front of such sheet, as shown in FIG. 1A. Luminescent elements are established at each region where a row and a column overlie each other so as to form connections on either side of the element. FIG. 2 represents each element as including both an LED aspect (indicated by a diode schematic symbol) and a parasitic capacitor aspect (indicated by a capacitor symbol labeled xe2x80x9cCPxe2x80x9d).
In operation, information is transferred to the matrix display by scanning each row in sequence. During each row scan period, each column connected to an element intended to emit light is also driven. For example, in FIG. 2 a row switch 228 grounds the row to which the cathodes of elements 222, 224 and 226 are connected during a scan of Row K. The column driver switch 272 connects the column connection 274 to the current source 270, such that the element 224 is provided with current. Each of the other columns 1 to N may also be providing current to the respective elements connected to Row K at this time, such as the elements 222 or 226. All current sources are typically at the same amplitude. OLED element light output is controlled by controlling the amount of time the current source for the particular column is on. When an OLED element has completed outputting light, its anode is pulled to ground to turn off the element. At the end of the scan period for Row K, the row switch 228 will typically disconnect Row K from ground and apply Vdd instead. Then, the scan of the next row will begin, with row switch 238 connecting the row to ground, and the appropriate column drivers supplying current to the desired elements, e.g. 232, 234 and/or 236.
Only one element (e.g. element 224) of a particular column (e.g. column J) is connected to each row (e.g. Row K), and hence only that element may be xe2x80x9cexposed,xe2x80x9d or connected to both the particular column drive (264) and row drive (228) so as to conduct current and luminesce during the scan of that row. However, each of the other devices on that particular column (elements 204, 214, 234 and 244 as shown, but actually including typically 63 other devices) are connected by the driver for their respective row (208, 218, 238 and 248 respectively) to a voltage source, Vdd. Therefore, the parasitic capacitance of each of the devices of the column is effectively in parallel with, or added to, the capacitance of the element being driven. The combined parasitic capacitance of the column limits the slew rate of a current drive such as drive 270 of column J. Yet, rapid driving of the elements is necessary. All rows must be scanned many times per second to obtain a reasonable visual appearance, which permits very little time for conduction for each row. Low slew rates may cause large exposure errors for short exposure periods. Thus, for practical implementations of display drivers using the prior art scheme, the parasitic capacitance of the columns may be a severe limitation on drive accuracy.
A luminescent device matrix and drive system as shown in FIG. 2 is described, for example, in U.S. Pat. No. 5,844,368 (Okuda et al.). To mitigate the effects of parasitic capacitances, Okuda suggests, for example, resetting each element between scans by applying either ground or Vcc (10V) to both sides of each element at the end of each exposure period. To initiate scanning a row, Okuda suggests conventionally connecting all unscanned rows to Vcc, and grounding the scanned row. An element being driven by a selected column line is therefore provided current from the parasitic capacitance of each element of the column line which is attached to an unscanned row. The Okuda patent does not reveal any means to establish the correct voltage for a selected element at the moment of turn-on. In many applications the voltage required for display elements at a given current will vary as a function of display manufacturing variations, display aging and ambient temperature, and Okuda also fails to provide any means to compensate for such variation.
The large parasitic capacitance of OLEDs in a matrix can cause substantial errors in the actual OLED current conducted in response to a controlled current drive. Accordingly, some form of precharge scheme is useful to bring the OLED elements of a matrix rapidly up to the voltage at which they will drive the intended current at the beginning of the row scan cycle. Moreover, since the voltage for an OLED varies substantially with temperature, process, and display aging, the light output of the display can be more accurately controlled if the xe2x80x9conxe2x80x9d voltage of the OLEDs is monitored or calibrated. Accordingly, what is needed in this industry is a means to determine and apply the correct voltage at the beginning of scans of current-driven devices in an array.
In response to the above-described need, a method and apparatus is provided for accurately monitoring or calibrating the display conduction voltages. The OLED response is so slow that the individual OLEDs may not be on long enough during a scan period to settle to their steady state voltage, making it difficult to monitor OLED voltages during an ordinary scan period. Accordingly, calibration may be performed during a calibration cycle.
In one aspect, the invention is a method for determining a precharge voltage for current-driven devices in a matrix. The method includes driving a selected current through a target device in the matrix, and determining an appropriate calibration time to measure a calibration voltage produced by the target device conducting the selected current. The appropriate calibration time is when the voltage produced in the target device by the selected current has reached steady state, and it may be determined by any of a number of different procedures, as elaborated in the detailed description. A voltage of the display is sampled at the calibration time, and a digital value created to represent the voltage is stored for later use during normal operation.
In another aspect, the invention is an apparatus for driving a current in an element of a display device. The apparatus includes two drivers, one for generating the current for the element, and another for connecting the other side of the element to a known voltage to accept the current. The apparatus also includes a sensing circuit to sense a voltage produced by the display device conducting a known current, and a precharge circuit configured to output a precharge voltage to the element based upon the sensed voltage.
In yet another aspect, the present invention is a method of calibrating a display device having at least one electroluminescent element and a display driver. The method includes applying a current to the element from a start time, and continuing the current for a predetermined period of time. At the end of the predetermined period, a display device voltage which reflects the element voltage is measured. After one or more measurement periods, a representation of the measured voltage is stored as a calibration value for later use during a non-calibration mode of the display device.
During normal operation the stored OLED voltage (Vcm) may be converted to an analog voltage by a digital to analog converter (DAC) and provided to each element during a column precharge period at the beginning of each scan cycle. After the precharge period, the channel output currents may be delivered to the channels in a conventional manner. At the end of the scan cycle, the individual columns may be shorted to ground in a conventional manner to terminate the element""s exposure time.